1. Field of the Invention
This invention relates to systems which collect data from multiple sensors, particularly accelerometers installed at various points around a manufacturing site, and bring the data to a central monitoring location. It is particularly related to systems where the sensors are uniquely addressable over a common data bus.
2. Description of Related Art
Production line and process machinery needs to be monitored for wear to avoid expensive, unexpected breakdowns. This is often done with accelerometers installed on the bearings of motors, pumps and other critical machinery to monitor the vibration of the equipment. Breakdowns are often preceded by changes in the equipment vibration level. These changes can be detected through regular monitoring, allowing the equipment to be repaired during regularly scheduled maintenance periods.
One method of monitoring the sensors is for maintenance personnel to take regular readings of the vibration levels at the sensors by carrying a portable data storage unit directly to each sensor to collect the data. This is time consuming, but effective.
Another method is for the sensors to be individually hardwired to a central station. In this arrangement, a separate wire pair runs from each sensor to the central monitoring station or to a multiplexer which is then connected to the central station. This, design simplifies the data collection task by allowing the data to be collected from the central station mounted in a convenient location.
In the central monitoring station design, other types of sensors, such as tachometers, pressure sensors, temperature sensors and the like may also be added to the system. This allows production process monitoring to be integrated into the vibration monitoring system.
In large systems, however, the numerous sensors makes the cost of running individual wire pairs from the sensors to the central location or multiplexer a major factor in the total cost of the system. Moreover, vibration sensors used to detect abnormal vibrations caused by wear, usually need to be monitored only briefly at widely scattered time intervals. Thus, prior art systems with a single wire pair running to each sensor has far more data transmitting capacity than is required for the majority of applications. A single data bus shared by the sensors is therefore desirable to reduce cabling cost and complexity.
Although the technique of using a common data bus for connecting to digitally addressable electronic elements is known, it has certain difficulties in a factory environment when implemented for monitoring sensors. One is that all the data from all the sensors passes over a single data bus. In the real world, with a network of sensors spread throughout a factory environment, a single data bus is susceptible to damage. An open circuit on the bus will cause the loss of information from all sensors beyond the open circuit and a short circuit makes communication impossible with any sensor on the bus.
Moreover, if the sensors are powered over the cable, the power transmission capabilities of the cable become a factor. Accelerometers are powered through the data bus wires over which the data is transmitted. Reasonably sized data bus wires do not have the power carrying capacity to continuously and simultaneously power the large number of sensors desired for many installations.
A further difficulty in a bus configuration design is that the sensors need to be uniquely identified by an address so that a desired sensor on the bus can be singled out and told to transmit its information. A digital address and digital address circuitry is clearly the most suitable for this task, but commonly available sensors generate analog data. This requires the use of one pair of wires for addressing the sensors and another set for transmitting the analog data. An alternative solution is to convert the analog data to digital form and run an all digital bus, but this requires expensive analog to digital circuitry at each sensor. Neither solution is desirable.
Accordingly, one object of the present invention is to provide a bused data collection system wherein the system includes redundant parallel data buses for fault tolerance in the event that any one bus is damaged.
Another object of the invention is to provide a design in which each sensor is connected to several data buses, and a central controller can electrically switch any sensor onto any selected one of the data buses.
A further object of the invention is to provide a design in which multiple parallel data buses may be simultaneously used to receive data from different sensors to permit real time direct comparison between the output of the sensors.
Still another object of the invention is to provide a design in which digital sensor addressing and analog data transmission occur on a shared data bus,
Another object of the invention is to provide a design in which multiple sensors share a single data bus, and that data bus is used bidirectionally to transmit digital address data from the central location to select a specific one of the sensors and to transmit analog sensor data from the sensor to the central location.
Still other objects and advantages of the invention will in part be obvious and will in pad be apparent from the specification.